This invention relates to the treatment of natural gas, and more particularly, to the treatment of natural gas to recover C.sub.2 + hydrocarbons.
Natural gas generally contains carbon dioxide, water vapor, ethane and heavier hydrocarbons, in addition to methane which comprises a major portion thereof. Ethane and the heavier hydrocarbons are generally extracted from natural gas streams, these streams are generally available at a pressure of from about 600 psig and above and often, it is necessary to send a residual methane gaseous stream at high pressure into a pipeline. When a high degree of hydrocarbon recovery is desired, a cryogenic process is invariably utilized requiring cooling and demethanization. Because of the thermodynamic properties of a natural gas stream, it is necessary to operate such a demethanizer at a pressure of from about 450 to 500 psia, which is substantially below the feed gas pressure and desired methane residual gas delivery pressure. Consequentially, the feed gas is depressurized and the methane residual gas is subsequently recompressed.
Heretofore, two basic processes have been employed for depressurizing the natural gas, i.e., (i) a Joule-Thomson throttling or (ii) an expansion through an expander. In the first mentioned process, the feed gas is chilled at high pressure and is then throttled through a valve. The methane residual gas is recompressed at least in part by a compressor driven by a steam turbine, electric motor or gas turbine. Such a process has several disadvantages, e.g., chilling of the natural gas is effected at high pressure and requires expensive, high pressure heat exchanges; the potential work of expansion is lost by throttling through a valve; and a special driver must be provided for the residual gas compressor.
The expansion processes either involve chilling in high pressure heat exchangers followed by expansion through a turboexpander, or expansion of the natural gas to the pressure of the demethanizer followed by chilling at the pressure of the demethanizer. Often, the expander is used to drive a compressor to partially recompress the methane residual gas. In both of such expander processes, an additional residual methane gas compressor including driver is usually required.
In many processes extracting ethane and heavier hydrocarbons, it is necessary to remove carbon dioxide from the natural gas feed to prevent freeze out of the carbon dioxide in the colder sections of the plant. This is generally effected by contacting the feed gas with an aqueous solution of an absorbent, such as monoethanolamine, and the like. Additionally, the natural gas feed has been passed through a drying step prior to introduction into the pipeline to thereby prevent hydrate formation in the pipeline. However, after contacting with an aqueous absorbent, further drying is required.